Limited swing driver circuit

ABSTRACT

A limited swing driver with a pass transistor coupled between a memory cell and an associated bitline; an inverter, its output coupled to the gate of the pass transistor, and its input coupled with the memory cell. A memory node is formed at the juncture of the inverter input and the memory cell forming a memory node. The driver also includes a discharge transistor coupled between the memory node and ground. The discharge transistor is driven by an input on the discharge transistor gate. It is preferred that the discharge transistor being programmed to produce a limited swing voltage at the memory node. It is desirable that the limited swing voltage be less than about 350 mV, and it is preferable that the limited swing voltage be between about 300 mV and about 200 mV. In addition, the limited swing voltage driver can include a tri-state output enable isolating the memory node from the bitline, particularly if the bitline is a shared or multiplexed bitline; and a self-reset circuit resetting the driver to a predetermined signal state.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] The present application claims the benefit of the filing dates ofthe following United States Provisional Patent Applications, thecontents of all of which are hereby expressly incorporated herein byreference:

[0002] Ser. No. 60/215,741, filed Jun. 29, 2000, and entitled MEMORYMODULE WITH HIERARCHICAL FUNCTIONALITY;

[0003] Ser. No. 60/193,607, filed Mar. 31, 2000, and entitled MEMORYREDUNDANCY IMPLEMENTATION;

[0004] Ser. No. 60/193,606, filed Mar. 31, 2000, and entitled DIFFUSIONREPLICA DELAY CIRCUIT;

[0005] Ser. No. 60/179,777, filed Feb. 2, 2000, and entitled SPLIT DUMMYBITLINES FOR FAST, LOW POWER MEMORY;

[0006] Ser. No. 60/193,605, filed Mar. 31, 2000, and entitled A CIRCUITTECHNIQUE FOR HIGH SPEED LOW POWER DATA TRANSFER BUS;

[0007] Ser. No. 60/179,766, filed Feb. 2, 2000, and entitled FASTDECODER WITH ASYNCHRONOUS RESET;

[0008] Ser. No. 60/220,567, filed Jul. 25, 2000, and entitled FASTDECODER WITH ROW REDUNDANCY;

[0009] Ser. No. 60/179,866, filed Feb. 2, 2000, and entitled HIGHPRECISION DELAY MEASUREMENT CIRCUIT;

[0010] Ser. No. 60/179,718, filed Feb. 2, 2000, and entitled LIMITEDSWING DRIVER CIRCUIT;

[0011] Ser. No. 60/179,765, filed Feb. 2, 2000, and entitledSINGLE-ENDED SENSE AMPLIFIER WITH SAMPLE-AND-HOLD REFERENCE;

[0012] Ser. No. 60/179,768, filed Feb. 2, 2000, and entitled SENSEAMPLIFIER WITH OFFSET CANCELLATION AND CHARGE-SHARE LIMITED SWINGDRIVERS; and

[0013] Ser. No. 60/179,865, filed Feb. 2, 2000, and entitled MEMORYARCHITECTURE WITH SINGLE PORT CELL AND DUAL PORT (READ AND WRITE)FUNCTIONALITY.

[0014] The following related patent applications, assigned to the sameassignee hereof and filed on even date herewith in the names of the sameinventors as the present application, disclose related subject matter,with the subject of each being incorporated by reference herein in itsentirety:

[0015] Memory Module with Hierarchical Functionality, Attorney DocketNo. 40050/B600/JFO; High Precision Delay Measurement Circuit, AttorneyDocket No. 37079/B600/JFO; Single-Ended Sense Amplifier withSample-and-Hold Reference, Attorney Docket No. 37362/B600/JFO; LimitedSwitch Driver Circuit, Attorney Docket No. 37361/B600/JFO; Fast Decoderwith Asynchronous Reset with Row Redundancy; Attorney Docket No.37115/B600/JFO; Diffusion Replica Delay Circuit, Attorney Docket No.37360/B600/JFO; Sense Amplifier with Offset Cancellation andCharge-Share Limited Swing Drivers, Attorney Docket No. 37363/B600/JFO;Memory Architecture with Single-Port Cell and Dual-Port (Read and Write)Functionality, Attorney Docket No. 37364/B600/JFO; Memory RedundancyImplementation, Attorney Docket No. 37496/B600/JFO; and; A CircuitTechnique for High Speed Low Power Data Transfer Bus, Attorney DocketNo. 37497/B600/JFO.

BACKGROUND OF THE INVENTION

[0016] 1. Field of the Invention

[0017] The present invention relates to memory devices, in particular,semiconductor memory devices, and most particularly, scalable,power-efficient semiconductor memory devices.

[0018] 2. Background of the Art

[0019] Memory structures have become integral parts of modern VLSIsystems, including digital signal processing systems. Although ittypically is desirable to incorporate as many memory cells as possibleinto a given area, memory cell density is usually constrained by otherdesign factors such as layout efficiency, performance, powerrequirements, and noise sensitivity.

[0020] In view of the trends toward compact, high-performance,high-bandwidth integrated computer networks, portable computing, andmobile communications, the aforementioned constraints can impose severelimitations upon memory structure designs, which traditional memorysystem and subcomponent implementations may fail to obviate.

[0021] One type of basic storage element is the static random accessmemory (SRAM), which can retain its memory state without the need forrefreshing as long as power is applied to the cell. In an SRAM device,the memory state II usually stored as a voltage differential within abistable functional element, such as an inverter loop. A SRAM cell ismore complex than a counterpart dynamic RAM (DRAM) cell, requiring agreater number of constituent elements, preferably transistors.Accordingly, SRAM devices commonly consume more power and dissipate moreheat than a DRAM of comparable memory density, thus efficient;lower-power SRAM device designs are particularly suitable for VLSIsystems having need for high-density SRAM components, providing thosememory components observe the often strict overall design constraints ofthe particular VLSI system. Furthermore, the SRAM subsystems of manyVLSI systems frequently are integrated relative to particular designimplementations, with specific adaptions of the SRAM subsystem limiting,or even precluding, the scalability of the SRAM subsystem design. As aresult SRAM memory subsystem designs, even those considered to be“scalable”, often fail to meet design limitations once these memorysubsystem designs are scaled-up for use in a VLSI system with need for agreater memory cell population and/or density.

[0022] There is a need for an efficient, scalable, high-performance,low-power memory structure that allows a system designer to create aSRAM memory subsystem that satisfies strict constraints for device area,power, performance, noise sensitivity, and the like.

SUMMARY OF THE INVENTION

[0023] The present invention satisfies the above needs by providing alimited swing driver, having a pass transistor coupled between a memorycell and an associated bitline; an inverter, the inverter output coupledto the gate of the pass transistor, and selectively driving the passtransistor and the inverter input coupled with the memory cell. A memorynode is formed at the juncture of the inverter input and the memory cellforming a memory node. The driver also includes a discharge transistorcoupled between the memory node and ground. The discharge transistorbeing driven by an input on the discharge transistor gate. It ispreferred that the discharge transistor being programmed to produce alimited swing voltage at the memory node. It is desirable that thelimited swing voltage be less than about 350 mV, and it is preferablethat the limited swing voltage be between about 300 mV and about 200 mV.In addition, the limited swing voltage driver can include a tri-stateoutput enable isolating the memory node from the bitline, particularlyif the bitline is a shared or multiplexed bitline; and a self-resetcircuit resetting the driver to a predetermined signal state.

[0024] The present invention will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following detailed description, appended claims and accompanyingdrawings, wherein:

[0026]FIG. 1 is a block diagram of an exemplary static random accessmemory (SRAM) architecture;

[0027]FIG. 2 is a general circuit schematic of an exemplarysix-transistor CMOS SRAM memory cell;

[0028]FIG. 3 is a block diagram of an embodiment of a hierarchicalmemory module using local bitline sensing, according to the presentinvention;

[0029]FIG. 4 is a block diagram of an embodiment of a hierarchicalmemory module using an alternative local bitline sensing structure;

[0030]FIG. 5 is a block diagram of an exemplary two-dimensional,two-tier hierarchical memory structure, employing plural local bitlinesensing modules of FIG.3;

[0031]FIG. 6 is a block diagram of an exemplary hierarchical memorystructure depicting a memory module employing both local word linedecoding and local bitline sensing structures;

[0032]FIG. 7 is a perspective illustration of a hierarchical memorystructure having a three-tier hierarchy, in accordance with theinvention herein;

[0033]FIG. 8 is a circuit schematic of an asynchronously-resettabledecoder, according to an aspect of the present invention;

[0034]FIG. 9 is a circuit schematic of a limited swing driver circuit,according to an aspect of the present invention;

[0035]FIG. 10 is a circuit schematic of a single-ended sense amplifiercircuit with sample-and-hold reference, according to an aspect of thepresent invention;

[0036]FIG. 11 is a circuit schematic of charge-share, limited-swingdriver sense amplifier circuit, according to an aspect of the presentinvention;

[0037]FIG. 12 is a block diagram illustrating an embodiment ofhierarchical memory module redundancy;

[0038]FIG. 13 is a block diagram illustrating another embodiment ofhierarchical memory module redundancy;

[0039]FIG. 14 is a block diagram of a memory redundancy device,illustrating yet another embodiment of hierarchical memory moduleredundancy;

[0040]FIG. 15A is a diagrammatic representation of the signal flow of anexemplary unfaulted memory module featuring column-oriented redundancy;

[0041]FIG. 15B is a diagrammatic representation of the shifted signalflow of the exemplary faulted memory module illustrated in FIG. 15A;

[0042]FIG. 16 is a generalized block diagram of a redundancy selectorcircuit, illustrating still another embodiment of hierarchical memorymodule redundancy;

[0043]FIG. 17 is a circuit schematic of an embodiment of a global rowdecoder having row redundancy according to the invention herein;

[0044]FIG. 18 is a block diagram illustrating dual-port functionality ina single-port hierarchical memory structure employing hierarchicalmemory modules according to the present invention;

[0045]FIG. 19 is a schematic diagram of one embodiment of a highprecision delay measurement circuit, according to the present invention;

[0046]FIG. 20 is a simplified block diagram of one aspect of the presentinvention employing one embodiment of a diffusion replica delay circuit;

[0047]FIG. 21 is a simplified block diagram of one aspect of the presentinvention employing another embodiment of a diffusion replica delaycircuit;

[0048]FIG. 22A is a schematic diagram of another aspect of an embodimentof the present invention, employing a high-speed, low-power datatransfer bus circuit; and

[0049]FIG. 22B is a schematic diagram of another aspect of an embodimentof the present invention, employing a high-speed, low-power datatransfer bus circuit.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] As will be understood by one having skill in the art, most VLSIsystems, including communications systems and DSP devices contain VLSImemory subsystems. Modern applications of VLSI memory subsystems almostinvariably demand high efficiency, high performance implementations thatmagnify the design tradeoff between layout efficient, speed, powerconsumption, scalability, design tolerances, and the like. The presentinvention ameliorates these tradeoffs using a novel hierarchicalarchitecture. The memory module of the present invention also can employone or more novel components which further add to the memory modulesefficiency and robustness.

[0051] Hereafter, but solely for the purposes of exposition, it will beuseful to describe the various aspects and embodiments of the inventionherein in the context of an SRAM memory structure, using CMOS SRAMmemory cells. However, it will be appreciated by those skilled in theart the present invention is not limited to CMOS-based processes andthat, mutatis mutandi, these aspects and embodiments may be used incategories of memory products other than SRAM, including withoutlimitation, DRAM, ROM, PLA, and the like, whether embedded within a VLSIsystem, or a stand alone memory device.

[0052] Examplary SRAM Module and Storage Cell

[0053]FIG. 1 is a functional block diagram of SRAM memory structure 100that illustrates the basic features of most SRAM subsystems. Module 100includes memory core 102, word line controller 104, precharge controller112, memory address inputs 114, and bitline controller 116. Memory core102 is composed of a two-dimensional array of K-bits of memory cells103, which is arranged to have C columns and R rows of bit storagelocations, where K=(C×R). The most common configuration of memory core102 uses single word line 106 to connect cells 103 onto paireddifferential bitlines 118. In general, core 102 is arranged as an arrayof 2^(P) word lines, based on a set of P memory address input lines 114i.e., R=2^(P). Thus, the p-bit address is decoded by row address decoder110 and column address decoder 122. Access to a given memory cell 103within such a single-core memory is accomplished by activating thecolumn 105 and the row 106 corresponding to cell 103. Column 105 isactivated by selecting, and switching, all bitlines in the particularcolumn corresponding to cell 103.

[0054] The particular row to be accessed is chosen by selectiveactivation of row address decoder 110, which usually correspondsuniquely with a given row, or word line, spanning all cells 103 on theparticular row. Also, word driver 108 can drive selected word line 106such that selected memory cell 103 can be written into or read out, on aparticular pair of bitlines 118, according to the bit address suppliedto memory address inputs 114.

[0055] Bitline controller 116 can include precharge cells 120, columnmultiplexers 122, sense amplifiers 124, and input/output buffers 126.Because differential read/write schemes are typically used for memorycells, it is desirable that bitlines be placed in a well-defined statebefore being accessed. Precharge cells 120 can be used to set up thestate of bitlines 118, through a PRECHARGE cycle, according to apredefined precharging scheme. In a static precharging scheme, prechargecells 120 can be left continuously on. While often simple to implement,static precharging can add a substantial power burden to active deviceoperation. Dynamic precharging schemes can use clocked precharge cells120 to charge the bitlines and, thus, can reduce the power budget ofstructure 100. In addition to establishing a defined state on bitlines118, precharging cells 120 can also be used to effect equalization ofdifferential voltages on bitlines 118 prior to a read operation. Senseamplifiers 124 allow the size of memory cell 103 to be reduced bysensing the differential voltage on bitline 118, which is indicative ofits state, and translating that differential voltage into a logic-leversignal.

[0056] In general a READ operation is performed by enabling row decoder110, which selects a particular row. The charge on one bitlines 118 fromeach pair of bitlines on each column will discharge through the enabledmemory cell 103, representing the state of the active cells 103 on thatcolumn 105. Column decoder 122 will enable only one of the columns, andwill connect bitlines 118 to input/output buffer 126. Sense amplifiers124 provide the driving capability to source current to input/outputbuffer 126. When sense amplifier 124 is enabled, the unbalanced bitlines118 will cause the balanced sense amplifier to trip toward the state ofthe bitlines, and data 125 will be output by buffer 126.

[0057] A WRITE operation is performed by applying data 125 to I/Obuffers 126. Prior to the WRITE operation, bitlines 118 are prechargedby precharge cells 120 to a predetermined value. The application ofinput data 125 to I/O buffers 126 tend to discharge the prechargevoltage on one of the bitlines 118, leaving one bitline logic HIGH andone bitline logic LOW. Column decoder 122 selects a particular column105 connecting bitlines 118 to I/O buffers 126, thereby discharging oneof the bitlines 118. The row decoder 110 selects a particular row, andthe information on bitlines 118 will be written on cell 103 at theintersection of column 105 and row 106. At the beginning of a typicalinternal timing cycle, precharging is disabled, and is not enabled againuntil the entire operation is completed. Column decoder 122 and rowdecoder 110 are then activated, followed by the activation of senseamplifier 124. At the conclusion of a READ or a WRITE operation, senseamplifier 124 is deactivated. This is followed by disabling decoders110, 122, at which time precharge cells 120 become active again during asubsequent PRECHARGE cycle. In general, keeping sense amplifier 124activated during the entire READ/WRITE operation leads to excessivedevice power consumption, because sense amplifier 124 needs to be activeonly for the actual time required to sense the state of memory cell 103.

[0058]FIG. 2 illustrates one implementation of memory cell 103 in FIG.1, in the form of six-transistor CMOS cell 200. Transistor cell 200 isone type of transistor which also may be used in embodiments of thepresent invention. SRAM cell 200 can be in one of three possible states:(1) the STABLE state, in which cell 200 holds a signal valuecorresponding to a logic “1” or logic “0”; (2) a READ operation state;or (3) a WRITE operation state. In the STABLE state, memory cell 200 iseffectively disconnected from the memory core (e.g., core 102 in FIG.1). Bitlines 202, 204 are precharged HIGH (logic “1”) before anyoperation (READ or WRITE) can take place. Row select transistors 206,208 are turned off during precharge. Precharge power is supplied byprecharge cells (not shown) coupled with the bitlines 202, 204, similarto precharge cells 120 in FIG. 1. A READ operation is initiated byperforming a PRECHARGE cycle, precharging bitlines 202, 204 to logicHIGH, and activating word line 205 using row select transistors 206,208. One of the bitlines 202, 204 discharges through bit cell 200, and adifferential voltage is setup between the bitlines 202, 204. Thisvoltage is sensed and amplified to logic levels. A WRITE operation tocell 200 is carried out after another PRECHARGE cycle, by drivingbitlines 202, 204 to the required state, and activating word line 205.CMOS is a desirable technology because the supply current drawn by suchan SRAM cell typically is limited to the leakage current of transistors201 a-d while in the STABLE state.

[0059] As memory cell density increases, and as memory components arefurther integrated into more complex systems, it becomes imperative toprovide memory architectures that are robust, reliable, fast, and area-and power-efficient. Single-core architectures, similar to thoseillustrated in FIG. 1, are increasingly unable to satisfy the power,speed, area and robustness constraints for a given high-performancememory application. Therefore, it is desirable to minimize powerconsumption, increase device speed, and improve device reliability androbustness, and numerous approaches have been developed to those ends.The advantages of the present invention may be better appreciated withinthe following context of some of these approaches, particularly as theyrelate to power reduction and speed improvement, and to redundancy androbustness.

[0060] Power Reduction and Speed Improvement

[0061] In reference to FIG. 1, the content of memory cell 103 of memoryblock 100 is detected in sense amplifier 102, using a differentialsignal between bitlines 104, 106. However, this architecture is notscalable. Also, as memory block 100 is made larger, there are practicallimitations to the ability of sense amplifier 102 to receive an adequatesignal in a timely fashion at bitlines 104, 106. Increasing the lengthof bitlines 104, 106, increases the associated bitline capacitance and,thus, increases the time needed for a signal to develop on bitlines 104,106. More power must be supplied to lines 104, 106 to overcome theadditional capacitance. Also, under the architectures of the existingart, it takes more time to precharge longer bitlines, thereby reducingthe effective device speed. Similarly, writing to longer bitlines 104,106, as found in the existing art, requires more extensive precharging,thereby increasing the power demands of the circuit, and furtherreducing the effective device speed.

[0062] In general, reduced power consumption in memory devices such asstructure 100 in FIG. 1 can be accomplished by, for example, reducingtotal switched capacitance, and minimizing voltage swings. Theadvantages of the power reduction aspects of certain embodiments of thepresent invention can further be appreciated within the context ofswitched capacitance reduction and voltage swing limitation.

[0063] Switched Capacitance Reduction

[0064] As the bit density of memory structures increases, it has beenobserved that single-core memory structures can have unacceptably largeswitching capacitances associated with each memory access. Access to anybit location within such a single-core memory necessitates enabling theentire row, or word line, in which the datum is stored, and switchingall bitlines in the structure. Therefore, it is desirable to designhigh-performance memory structures to reduce the total switchedcapacitance during any given access.

[0065] Two well-known approaches for reducing total switched capacitanceduring a memory structure access include dividing a single-core memorystructure into a banked memory structure, and employing divided wordline structures. In the former approach, it is necessary to activateonly the particular memory bank associated with the memory cell ofinterest. In the latter approach, total switched capacitance is reducedby localizing word line activation to the greatest practicable extent.

[0066] Divided or Banked Memory Core

[0067] One approach to reducing switching capacitances is to divide thememory core into separately switchable banks of memory cells. Typically,the total switched capacitance during a given memory access for bankedmemory cores is inversely proportional to the number of banks employed.By judiciously selecting the number and placement of bank units within agiven memory core design, as well as the type of decoding used, thetotal switching capacitance, and thus the overall power consumed by thememory core, can be greatly reduced. A banked design also may realize ahigher product yield, because the memory banks can be arranged such thata defective bank is rendered inoperable and inaccessible, while theremaining operational banks of the memory core can be packed into alower-capacity product.

[0068] However, banked designs may not be appropriate for certainapplications. Divided memory cores demand additional decoding circuitryto permit selective access to individual banks, and incur a delay as aresult. Also, many banked designs employ memory segments that are merelyscaled-down versions of traditional monolithic core memory designs, witheach segment having dedicated control, precharging, decoding, sensing,and driving circuitry. These circuits tend to consume much more power inboth standby and operational modes, than do their associated memorycells. Such banked structures may be simple to design, but theadditional complexity and power consumption thus can reduce overallmemory component performance.

[0069] By their very nature, banked designs are not suitable forscaling-up to accommodate large design requirements. Also, traditionalbanked designs may not be readily conformable to applications requiringa memory core configuration that is substantially different from theunderlying memory bank architecture (e.g., a memory structure needingrelatively few rows of very long bit-length word lengths). Rather thanresort to a top-down division of the basic memory structure using bankedmemory designs, preferred embodiments of the present invention provide ahierarchical memory structure that is synthesized using a bottom-upapproach, by hierarchically coupling basic memory modules with localizeddecision-making features that synergistically cooperate to dramaticallyreduce the overall power needs, and improve the operating speed, of thestructure. At a minimum, such a basic hierarchical module can includelocalized bitline sensing.

[0070] Divided Word Line

[0071] Often, the bit-width of a memory component is sized toaccommodate a particular word length. As the word length for aparticular design increases, so do the associated word line delays,switched capacitance, power consumption, and the like. To accommodatevery long word lines, it may be desirable to divide core-spanning globalword lines into local word lines, each consisting of smaller groups ofadjacent, word-oriented memory cells. Each local group employs localdecoding and driving components to produce the local word line signalswhen the global word line, to which it is coupled, is activated. In longword length applications, the additional overhead incurred by dividedword lines can be offset by reduced word line delays, power consumptionand so forth. However, the added overhead imposed by existing dividedword line schemes may make it unsuitable for many implementations. Asbefore, rather than resorting to the traditional top-down division ofword lines, certain preferred embodiment of the invention herein includeproviding a local word line to the aforementioned basic memory module,which further enhances the local decision making features of the module.As before, by using a bottom-up approach to hierarchically couple basicmemory modules, here with the added localized decision-making featuresof local word lines according to the present invention, additionalsynergies are realized, which further reduce overall power consumptionand signal propagation times.

[0072] Voltage-Swing Reduction Techniques

[0073] Power reduction also can be achieved by reducing the voltageswings experienced throughout the structure. By limiting voltage swings,it is possible to reduce the amount of power dissipated as the voltageat a node or on a line decays during a particular event or operation, aswell as to reduce the amount of power required to return the variousdecayed voltages to the desired state after the particular event oroperation, or prior to the next access. Two techniques to this endinclude using pulsed word lines and sense amplifier voltage swingreduction.

[0074] Pulsed Word Lines

[0075] By enabling a word line just long enough to correctly detect thedifferential voltage across a selected memory cell, it is possible toreduce the bitline voltage discharge corresponding to a READ operationon the selected cell. In some designs, by applying a pulsed signal tothe associated word line over a chosen interval, a sense amplifier isactivated only during that interval, thereby reducing the duration ofthe bitline voltage decay. These designs typically use some form ofpulse generator that produces a fixed-duration pulse. If the duration ofthe pulse is targeted to satisfy worst-case timing scenarios, theadditional margin will result in unnecessary bitline current draw duringnominal operations. Therefore, it is desirable to employ a self-timed,self-limiting word line device that is responsive to the actual durationof a given READ operation on a selected cell, and that substantiallylimits word line activation to that duration. Furthermore, where a senseamplifier can successfully complete a READ operation in less than amemory system clock cycle, it also may be desirable that the pulse widthactivation be asynchronous, relative to the memory system clock. Certainaspects of the present invention provide a pulsed word line signal, forexample, using a cooperative interaction between global and local wordline decoders.

[0076] Sense Amplifier Voltage Swing Reduction

[0077] In order to make large memory arrays, it is most desirable tokeep the size of an individual memory cell to a minimum. As a result,individual memory cells generally are incapable of supplying drivingcurrent to associated input/output bitlines. Sense amplifiers typicallyare used to detect the value of the datum stored in a particular memorycell and to provide the current needed to drive the I/O lines. In senseamplifier design, there typically is a trade-off between power andspeed, with faster response times usually dictating greater powerrequirements. Faster sense amplifiers can also tend to be physicallylarger, relative to low speed, low power devices. Furthermore, theanalog nature of sense amplifiers can result in their consuming anappreciable fraction of the total power. Although one way to improve theresponsiveness of a sense amplifier is to use a more sensitive senseamplifier, any gained benefits are offset by the concomitant circuitcomplexity which nevertheless suffers from increased noise sensitivity.It is desirable, then, to limit bitline voltage swings and to reduce thepower consumed by the sense amplifier.

[0078] In one typical design, the sense amplifier detects the smalldifferential signals across a memory cell, which are in an unbalancedstate representative of datum value stored in the cell, and amplifiesthe resulting signal to logic level. Prior to a READ operation, thebitlines associated with a particular memory column are precharged to achosen value. When a specific memory cell is enabled, a row decoderselects the particular row in which the memory cell is located, and anassociated column decoder selects a sense amplifier associated with theparticular column. The charge on one of those bitlines is dischargedthrough the enabled memory cell, in a manner corresponding to the valueof the datum stored in the memory cell. This produces an imbalancebetween the signals on the paired bitlines, and causing a bitlinevoltage swing. When enabled, the sense amplifier detects the unbalancedsignal and, in response, the usually-balanced sense amplifier statechanges to a state representative of the value of the datum. This statedetection and response occurs within a finite period, during which aspecific amount of power is dissipated. The longer it takes to detectthe unbalanced signal, the greater the voltage decay on the prechargedbitlines, and the more power dissipated during the READ operation. Anypower that is dissipated beyond the actual time necessary for sensingthe memory cell state, is truly wasted power. In traditional SRAMdesigns, the sense amplifiers that operate during a particular READoperation, remain active during nearly the entire read cycle. However,this approach unnecessarily dissipates substantial amounts of power,considering that a sense amplifier needs to be active just long enoughto correctly detect the differential voltage across a selected memorycell, indicating the stored memory state.

[0079] There are two general approaches to reducing power in senseamplifiers. First, sense amplifier current can be limited by using senseamplifiers that automatically shut off once the sense operation hascompleted. One sense amplifier design to this end is a self-latchingsense amplifier, which turns off as soon as the sense amplifierindicates the sensed datum state. Second, sense amplifier currents canbe limited by constraining the activation of the sense amplifier toprecisely the period required. This approach can be realized through theuse of a dummy column circuit, complete with bit cells, sense amplifier,and support circuitry. By mimicking the operation of a functionalcolumn, the dummy circuit can provide to a sense amplifier timingcircuit an approximation of the activation period characteristic of thefunctional sense amplifiers in the memory system. Although the dummycircuit approximation can be quite satisfactory, there is an underlyingassumption that all functional sense amplifiers have completed thesensing operation by the time the dummy circuit completes the itsoperation. In that regard, use of a dummy circuit can be similar toenabling the sense amplifiers with a fixed-duration pulsed signal.Aspects of the present invention provide circuitry and sense amplifierswhich limit voltage swings, and which improve the sensitivity androbustness of sense amplifier operation. For example, compact,power-conserving sense amplifiers having increased immunity to noise, aswell as to intrinsic and operational offsets, are provided. In thecontext of the present invention, such sense amplifiers can be realizedat the local module tier, as well as throughout the higher tiers of ahierarchical memory structure, according to the present invention.

[0080] Redundancy

[0081] Memory designers typically balance power and device area againstspeed. High-performance memory components place a severe strain on thepower and area budgets of associated systems particularly where suchcomponents are embedded within a VLSI system, such as a digital signalprocessing system. Therefore, it is highly desirable to provide memorysubsystems that are fast, yet power-and area-efficient. Highlyintegrated, high performance components require complex fabrication andmanufacturing processes. These processes experience unavoidableparameter variations which can impose physical defects upon the unitsbeing produced, or can exploit design vulnerabilities to the extent ofrendering the affected units unusable, or substandard.

[0082] In a memory structure, redundancy can be important, for example,because a fabrication flaw, or operational failure, of even a single bitcell may result in the failure of the system relying upon the memory.Likewise, process invariant features may be needed to insure that theinternal operations of the structure conform to precise timing andparametric specifications. Lacking redundancy and process invariantfeatures, the actual manufacturing yield for a particular memorystructure can be unacceptably low. Low-yield memory structures areparticularly unacceptable when embedded within more complex systems,which inherently have more fabrication and manufacturingvulnerabilities. A higher manufacturing yield translates into a lowerper-unit cost and robust design translates into reliable products havinglower operational costs. Thus, it is also highly desirable to designcomponents having redundancy and process invariant features whereverpossible.

[0083] Redundancy devices and techniques constitute other certainpreferred aspects of the invention herein which, alone or together,enhance the functionality of the hierarchical memory structure. Theaforementioned redundancy aspects of the present invention can renderthe hierarchical memory structure less susceptible to incapacitation bydefects during fabrication or during operation, advantageously providinga memory product that is at once more manufacturable and cost-efficient,and operationally more robust. Redundancy within a hierarchical memorymodule can be realized by adding one or more redundant rows, columns, orboth, to the basic module structure. In one aspect of the presentinvention a decoder enabling row redundancy is provided. Moreover, amemory structure composed of hierarchical memory modules can employ oneor more redundant modules for mapping to failed memory circuits. Aredundant module can provide a one-for-one replacement of a failedmodule, or it can provide one or more memory cell circuits to one ormore primary memory modules.

[0084] Memory Module with Hierarchical Functionality

[0085] The modular, hierarchical memory architecture according to theinvention herein provides a compact, robust, power-efficient,high-performance memory system having, advantageously, a flexible andextensively scalable architecture. The hierarchical memory structure iscomposed of fundamental memory modules which can be cooperativelycoupled, and arranged in multiple hierarchical tiers, to devise acomposite memory product having arbitrary column depth or row length.This bottom-up modular approach localizes timing considerations,decision making, and power consumption to the particular unit(s) inwhich the desired data is stored.

[0086] Within a defined design hierarchy, the fundamental memory modulescan be grouped to form a larger memory block, that itself can be coupledwith similar memory structures to form still larger memory blocks. Inturn, these larger structures can be arranged to create a complexstructure at the highest tier of the hierarchy. In hierarchical sensing,it is desired to provide two or more tiers of bit sensing, therebydecreasing the read and write time of the device, i.e., increasingeffective device speed, while reducing overall device powerrequirements. In a hierarchical design, switching and memory cell powerconsumption during a read/write operation are localized to the immediatevicinity of the memory cells being evaluated or written, i.e., thosememory cells in selected memory modules, with the exception of a limitednumber of global word line selectors and sense amplifiers, and supportcircuitry. The majority of modules that do not contain the memory cellsbeing evaluated or written generally remain inactive.

[0087] Preferred embodiments of the present invention provide ahierarchical memory module using local bitline sensing, local word linedecoding, or both, which intrinsically reduces overall power consumptionand signal propagation, and increases overall speed, as well as designflexibility and scalability. Aspects of the present inventioncontemplate apparatus and methods which further limit the overall powerdissipation of the hierarchical memory structure, while minimizing theimpact of a multi-tier hierarchy. Certain aspects of the presentinvention are directed to mitigate functional vulnerabilities that maydevelop from variations in operational parameters, or that related tothe fabrication process. In addition, devices and techniques aredisclosed which advantageously ameliorate system performance degradationresulting from temporal inefficiencies, including, without limitation, ahigh-precision delay measurement circuit, a diffusion delay replicationcircuit and associated dummy devices. In another aspect of the presentinvention, an asynchronously resettable decoder is provided that reducesthe bitline voltage discharge, corresponding, for example, to a READoperation on the selected cell, by limiting word-line activation to theactual time required for the sense amplifier to correctly detect thedifferential voltage across a selected memory cell.

[0088] Hierarchical Memory Modules

[0089] In prior art memory designs, such as the aforementioned bankeddesigns, large logical memory blocks are divided into smaller, physicalmodules, each having the attendant overhead of an entire block of memoryincluding predecoders, sense amplifiers, multiplexers, and the like. Inthe aggregate, such memory blocks would behave as an individual memoryblock. However, using the present invention, memory blocks ofcomparable, or much larger, size can be provided by couplinghierarchical functional modules into larger physical memory blocks ofarbitrary number of words and word length. For example, existing designswhich aggregate smaller memory blocks into a single logical blockusually require the replication of the predecoders, sense amplifiers,and other overhead circuitry that would be associated with a singlememory block. According to the present invention, this replication isunnecessary, and undesirable. One embodiment of the inventioncomprehends local bitline sensing, in which a limited number of memorycells are coupled with a single local sense amplifier, thereby forming abasic memory module. Similar memory modules are grouped and arranged tooutput the local sense amplifier signal to the global sense amplifiersignal. Thus, the bitlines associated with the memory cells are notdirectly coupled with a global sense amplifier, mitigating the signalpropagation delay and power consumption typically associated with globalbitline sensing. In this approach, the local bitline sense amplifierquickly and economically sense the state of a selected memory cell andreport the state to the global sense amplifier. In another embodiment ofthe invention herein, the delays and power consumption of global wordline decoding are mitigated by providing a memory module, composed of alimited number of memory cells, having local word line decoding. Similarto the local bitline sensing approach, a single global word line decodercan be coupled with the respective local word line decoders of multiplemodules. When the global decoder is activated with an address, only thelocal word line decoder associated with the desired memory cellresponds, and activates the memory cell. This aspect, too, isparticularly power-conservative and fast, because the loading on theglobal line is limited to the associated local word line decoders, andthe global word line signal need be present only as long as required totrigger the relevant local word line. In yet another embodiment of thepresent invention, a hierarchical memory module employing both localbitline sensing and local word line decoding is provided, which realizesthe advantages of both approaches. Each of the above embodiments arediscussed forthwith.

[0090] Local Bitline Sensing

[0091]FIG. 3 illustrates a memory block 300 formed by coupling multiplecooperating constituent modules 320 a-e, with each of the modules 320a-e having a respective local sense amplifier 308 a-e. Each module iscomposed of a predefined number of memory cells 325 a-g, which arecoupled with one of the respective local sense amplifiers 308 a-e. Eachlocal sense amplifiers 308 a-e is coupled with global sense amplifier302 via bitlines 304, 306. Because each of local sense amplifiers 308a-e sense only the local bitlines 310 a-e, 312 a-e, of the respectivememory modules 320 a-e, the amount of time and power necessary toprecharge local bitlines 310 a-e and 312 a-e are substantially reduced.Only when local sense amplifier 308 a-e senses a signal on respectivelocal lines 310 a-e and 312 a-e, does it provide a signal to globalsense amplifier 302. This architecture adds flexibility and scalabilityto a memory architecture design because the memory size can be increasedby adding locally-sensed memory modules such as 320 a-e.

[0092] Increasing the number of local sense amplifiers 308 a-e attachedto global bitlines 304, 306, does not significantly increase the loadingupon the global bitlines, or increase the power consumption in globalbitlines 304, 306 because signal development and precharging occur onlyin the local sense amplifier 308 a-e, proximate to the signal found inthe memory cells 325 a-g within corresponding memory module 320 a-e.

[0093] In preferred embodiments of the invention herein, it is desirableto have each module be self-timed. That is, each memory module 320 a-ecan have internal circuitry that senses and establishes a sufficientperiod for local sensing to occur. Such self-timing circuitry iswell-known in the art. In single-core designs, or even banked designs,self-timing memory cores may be unsuitable for high-performanceoperation, because the timing tends to be dependent upon the slowest ofmany components in the structure, and because the signal propagationtimes in such large structures can be significant. The implementation ofself-timing in these larger structures can be adversely affected byvariations in fabrication and manufacturing processes, which cansubstantially impact the operational parameters of the memory array andthe underlying timing circuit components.

[0094] In a hierarchical memory module, self-timing is desirable becausethe timing paths for each module 320 a-e comprehends only a limitednumber of memory cells 325 a-g over a very limited signal path. Eachmodule, in effect, has substantial autonomy in deciding the amount oftime required to execute a given PRECHARGE, READ, or WRITE operation.For the most part, the duration of an operation is very brief at thelocal tier, relative to the access time of the overall structure, sothat memory structure 300 composed of hierarchical memory modules 320a-e is not subject to the usual difficulties associated withself-timing, and also is resistant to fabrication and manufacturingprocess variations.

[0095] In general, the cores of localized sense amplifiers 308 a-e canbe smaller than a typical global sense amplifier 302, because arelatively larger signal develops within a given period on the localsense amplifier bitlines, 310 a-e, 312 a-e. That is, there is moresignal available to drive local sense amplifier 308 a-e. In aglobal-sense-amplifier-only architecture, a greater delay occurs while asignal is developed across the global bitlines, which delay can bedecreased at the expense of increased power consumption. Advantageously,local bit sensing implementations can reduce the delay whilesimultaneously reducing consumed power.

[0096] In certain aspects of the invention herein, detailed below, alimited swing driver signal can be sent from the active local senseamplifier to the global sense amplifier. A full swing signal also may besent, in which case, a very simple digital buffer, may be used. However,if a limited swing signal is used, a more complicated sense amplifiermay be needed. For a power constrained application, it may be desirableto share local sense amplifiers among two or more memory modules. Senseamplifier sharing, however, may slightly retard the bit signaldevelopment line indirectly because, during the first part of a sensingperiod, the capacitances of each of the top and the bottom shared memorymodules are being discharged. However, this speed decrease can beminimized and is relatively small, when compared to the benefits gainedby employing logical sense amplifiers over the existing global-onlyarchitectures. Moreover, preferred embodiments of the invention hereincan obviate these potentially adverse effects of sense amplifier sharingby substantially isolating the local sense amplifier from associatedlocal bitlines which are not coupled with the memory cell to be sensed.

[0097]FIG. 4 shows a memory structure 400, which is similar to structure300 in FIG. 3, by providing local bitline sensing of modules 420 a-d.Each memory module 420 a-d is composed of a predefined number of memorycells 425 a-g. Memory cells 425 a-g are coupled with respective localsense amplifier 408 a, b via local bitlines 410 a-d, 412 a-d. Unlikestructure 300 in FIG. 3, where each module 320 a-e has its own localsense amplifier 308 a-e, memory modules 420 a-d are paired with a singlesense amplifier 408 a, b. Similar to FIG. 3, FIG. 4 shows global senseamplifier 402 being coupled with local sense amplifiers 408 a, 408 b.

[0098]FIG. 5 further illustrates that memory structures such as module300 in FIG. 3 can be coupled such that the overall structure is extendedin address size (this is vertically), or in bit length (this ishorizontally), or both. The arrayed structure in FIG. 5 also can usemodules such as module 400 in FIG. 4. FIG. 5 also illustrates that acomposite memory structure 500 using hierarchical memory modules can betruly hierarchical. Memory blocks 502, 503 can be composed of multiplememory modules, such as module 504, which can be modules as described inreference to FIG. 3 and FIG. 4. Each memory block 502, 503 employstwo-tier sensing, as previously illustrated. However, in structure 500,memory blocks 502, 503 employ an intermediate tier of bitline sensing,using, for example, midtier sense amplifiers 514, 516. Under thehierarchical memory paradigm, midtier sense amplifiers 514, 516 can becoupled with global sense amplifier 520. Indeed, the hierarchical memoryparadigm, in accordance with the present invention, can comprehend ahighly-scalable multi-tiered hierarchy, enabling the memory designer todevise memory structures having memory cell densities and configurationsthat are tailored to the application. Advantageously, this scalabilityand configurability can be obtained without the attendant delays, andsubstantially increased power and area consumption of prior art memoryarchitectures.

[0099] One of the key factors in designing a faster, power-efficientdevice is that the capacitance per unit length of the global bitline canbe made less than the capacitance of the local bitlines. This isbecause, using the hierarchical scheme, the capacitance of the globalbitline is no longer constrained by the cell design. For example, metallines can be run on top of the memory device. Also, a multiplexingscheme can be used that increase the pitch of the bitlines, therebydispersing them, further reducing bitline capacitance. Overall, thedistance between the global bitlines can be wider, because the memorycells are not directly connected to the global bitlines. Instead, eachcell, e.g. cell 303 in FIG. 3., is connected only to the local senseamplifier, e.g. sense amplifier 308 a-e.

[0100] Local Word Line Decoding

[0101]FIG. 6 illustrates a hierarchical structure 600 havinghierarchical word-line decoding in which each hierarchical memory module605 is composed of a predefined number of memory cells 610, which arecoupled with a particular local word line decoder 615 a-c. Each localword line decoder 615 a-c is coupled with a respective global word linedecoder 620. Each global word line decoder 620 a-d is activated whenpredecoder 622 transmits address information relevant to a particularglobal word line decoder 620 a-d via predecoder lines 623. In response,global word line decoder 620 a-d activates global word line 630 which,in turn, activates a particular local word line decoder 615 a-c. Localword line decoder 615 a-c then enables associated memory module 605, sothat the particular memory cell 610 of interest can be evaluated. Eachof memory modules 605 can be considered to be an independent memorycomponent to the extent that the hierarchical functionality of each ofmodules 605 relies upon local sensing via local sense amplifiers 608a-b, local decoding via local word line decoders 615 a-c, or both. Aswith other preferred embodiments of the invention herein, it isdesirable to have each module 605 be self-timed. Self-timing can beespecially useful when used in conjunction with local word line decodingbecause a local timing signal from a respective one of memory module 605can be used to terminate global word line activation, local bitlinesensing, or both.

[0102] Similar to the scaling illustrated in FIG. 5, multiple memorydevices 600 can be arrayed coupled with global bitlines or globaldecoding word lines, to create a composite memory component of a desiredsize and configuration. In an embodiment of the present invention, 256rows of memory are used in each module 605, allowing the memory designerto create a memory block of arbitrary size, having a 256 rowgranularity. For prior art memory devices, a typical realisticlimitation to the number of bits sense per sense amplifier is about 512bit. Long bit or word lines can present a problem, particularly for aWRITE operations, because the associated driver can be limited by theamount of power it can produce, and the speed at which sufficient chargecan be built-up upon signal lines, such as global bitlines 604, 606 inFIG. 6.

[0103] Although FIG. 6 shows hierarchical word line decoding used inconjunction with hierarchical bitline operations, hierarchical word-linedecoding can be implemented without hierarchical bitline sensing. It ispreferred to use both the hierarchical word line decoding, and thehierarchical bitline sensing to obtain the synergistic effects ofdecreased power and increased speed for the entire device.

[0104] Hierarchical Functionality

[0105] In typical designs, power intends to increase approximatelylinearly with the size of the memory. However, according to the presentinvention, as illustrated in FIG. 3 through FIG. 6, power requirementsmay increase only fractionally as the overall memory structure sizeincreases, primarily because only the memory module, and associatedlocal bitlines and local word lines are activated during a givenoperation. Due to the localized functionality, the global bitlines andword lines are activated for relatively brief periods at the beginningand end of the operation. In any event, power consumption is generallydictated by the bit size of the word, and the basic moduleconfiguration, i.e., the number of rows and row length of modules 620a-e. Thus, significant benefits can be realized by judiciously selectingthe configuration of a memory module, relative to the overall memorystructure configuration. For example, in a memory structure according tothe present invention, a doubling in the size of the memory device canaccount for power consumption increase of about twenty percent, and nota doubling, as found in prior art designs. Furthermore, a memorystructure according to the present invention can realize afour-to-six-fold decrease in power requirements and can operate 30% to50% faster, and often more, than traditional architectures.

[0106]FIG. 7 illustrates that memory structures according to the presentinvention, for example memory structure 740, are fully hierarchical, inthat each tier within the hierarchy includes local bit line sensing,local word line decoding, or both. Exemplary memory structure 740 isthree-tier hierarchical device with memory module 700 beingrepresentative of the fundamental, or lowest, tier (L₀) of the memoryhierarchy; memory device 720 being representative of the intermediatetier(L₁) of the memory hierarchy; and memory structure 740 beingrepresentative of the upper tier (L₂) of the memory hierarchy. For thesake of simplicity, only one memory column is shown at each tier, suchthat memory column 702 is intended to be representative of fundamentaltier (L₀), memory column 722 of intermediate tier(L₁), and memory column742 of upper tier (L₂).

[0107] Tier L₀ memory devices, such as memory module 700, are composedof multiple memory cells, generally indicated by memory cell 701, whichcan be disposed in row, column, or 2-D array (row and column) formats.Memory module 700 is preferred to employ local bit line sensing, localword line decoding, or both, as was described relative to FIGS. 3through 6. In the present example, module M00 includes both local bitline sensing and local word line decoding. Each memory cell M01 in arespective column of memory cells 702 is coupled with local senseamplifier 703 by local bit lines 704 a, 704 b. Although local bit linesensing can be performed on a memory column having a single memory cell,it is preferred that two, or more, memory cells 701 be coupled withlocal sense amplifier 703. Unlike some prior art memory devices whichdispense with local bit line sensing by employing special memory cellswhich provide strong signals at full logic levels, module 700 can use,and indeed is preferred to use, conventional and low-power memory cells701 as constituent memory cells. An advantage of local bit line sensingis that only a limited voltage swing on bit lines 704 a, 704 b may beneeded by local sense amplifier 703 to accurately sense the state ofmemory cell 701, which permits rapid memory state detection andreporting using substantially less power than with prior art designs.

[0108] Tier Lo local sense amplifier 703 detects the memory state ofmemory cell 701 by coupling the memory state signal to tier L₀ localsense amplifier 703, via local bit lines 704 a, 704 b. It is preferredthat the memory state signal be a limited swing voltage signal.Amplifier 703 transmits a sensed signal representative of the memorystate of memory cell 701 to tier L₁ sense amplifier 723 via tier Lolocal sense amplifier outputs 705 a, 705 b, which are coupled withintermediate tier bit lines 724 a, 724 b. It is preferred that thesensed signal be a limited swing voltage signal, as well. In turn,amplifier 723 transmits a second sensed signal representative of thememory state of memory cell 701 to tier L₂ sense amplifier 743, via tierL₁ local sense amplifier outputs 725 a, 725 b, which are coupled withupper tier bit lines 744 a, 744 b. It also is preferred that the secondsensed signal be a limited voltage swing signal.

[0109] Where tier L₂ is the uppermost tier of the memory hierarchy, asis illustrated in the instant example, sense amplifier 743 can be aglobal sense amplifier, which propagates a third signal representativeof memory cell 701 to associated I/O circuitry (not shown)via senseamplifier output lines 746 a, 746 b. Such I/O circuitry can be similarto I/O in FIG. 1. However, the present invention contemplates ahierarchical structure that can consist of two, three, four, or more,tiers of hierarchy. The uppermost tier signal can be a full-swingsignal. In view of FIG. 7, a skilled artisan would realize that “localbit line sensing” occurs at each tier L₀, L₁, and L2, in the exemplaryhierarchy, and is desirable, for example, because only a limited voltageswing may be needed to report the requested memory state from a lowertier in the hierarchy to the next higher tier.

[0110] Hierarchical memory structures also can employ local word linedecoding, as illustrated in memory device 740. In FIG. 7, memory device740 is the uppermost tier (L₂) in the hierarchical memory structure,thus incoming global word line signal 746 is received from global wordline drivers (not shown) such as global row address decoders 110 inFIG. 1. In certain preferred embodiments of the present invention,predecoding is employed to effect rapid access to desired word lines,although predecoding is not required, and may not be desired, at everytier in a particular implementation. Signal M46 is received by uppertier predecoder 747, predecoded and supplied to upper tier (L₂) globalword line decoders, such as global word line decoder 748. Decoder M48 iscoupled with local word line decoder 749 by way of upper tier globalword line 750, and selectively activates upper tier local word linedecoder 749. Activated L₂ local decoder M49, in turn, activates L₂ localword line 751, which propagates selected word line signal 726 tointermediate tier (L₁) predecoder 727. Predecoder 727 decodes andactivates the appropriate intermediate tier (L₁) global word linedecoder, such as global word line decoder 728. Decoder 728 is coupledwith, and selectively activates, tier L₁ local word line decoder 729 byway of tier (L₁) global word line 730. Activated L₁ local decoder 729,in turn, propagates a selected word line signal 706 to fundamental tier(L₀) predecoder 707, which decodes and activates the appropriate tier L₀global word line decoder, such as global word line decoder 708.Activated L₀ local decoder 709, in turn, activates L₀ local word line711, and selects memory cell 701 for access. In view of the foregoingdiscussion of hierarchical word line decoding, a skilled artisan wouldrealize that “local word line decoding” occurs at each tier L₀, L₁, andL₂ in the exemplary hierarchy, and is desirable because a substantialreduction in the time and power needed to access selected memory cellscan be realized.

[0111] Although local word line decoding within module 700 is shown inthe context of a single column of memory cells, such as memory columns702, 722, 742, the present invention contemplates that local word linedecoding be performed across two, or more, columns in each of hierarchytiers, with each of the rows in the respective columns employing two ormore local word line decoders, such as local word line decoders 709,729, 749 which are coupled with respective global word line decoders,such as global word line decoders 708, 728, 748 by way of respectiveglobal word lines, such as global word lines 710, 730, 750. However,there is no requirement that equal numbers of rows and columns beemployed at any two tiers of the hierarchical structure. In general,memory device 720 can be composed of multiple memory modules 700, whichfundamental modules 700 can be disposed in row, column, or 2-D array(row and column) array formats. Such fundamental memory modules can besimilar to those illustrated with respect to FIG. 3 through FIG. 6, andcombinations thereof. Likewise, memory device 740 can be composed ofmultiple memory devices 720, which intermediate devices 720 also can bedisposed in row, column, or 2-D array (row and column) formats. Thisextended, and extendable, hierarchality permits the formation ofmultidimensional memory modules that are distinct from prior arthierarchy-like implementations, which generally are 2-D groupings ofbanked, paged, or segmented memory devices, or register file memorydevices, lacking local functionality at each tier in the hierarchy.

[0112] Fast Decoder with Asynchronous Reset

[0113] Typically, local decoder reset can be used to generate narrowpulse widths on word lines in a fast memory device. The input signals tothe word line decoder are generally synchronized to a clock, or chipselect, signal. However, it is desirable that the word line be resetindependently of the clock and also of the varying of the input signalsto the word line decoder.

[0114]FIG. 8 is a circuit diagram illustrative of anasynchronously-resettable decoder 800 according to this aspect of thepresent invention. It may be desirable to implement the AND function,for example, by source-coupled logic. The capacitance on the input x2_n802 can be generally large, therefore the AND function is performed withabout one inverter delay plus three buffer stages. The buffers areskewed, which decreases the load capacitance by about one-half anddecreases the buffer delay.

[0115] In order to be able to independently reset word line WL 804, itis desirable that inputs 802, 803 be isolated from output 804, and thenode 805 should be charged to V_(dd), turning off the large PMOS driverM8 807 once word line WL 804 is set to logical HIGH. Charging of node805 to V_(dd) can be accomplished by a feedback-resetting loop. Inputs802, 803 can be isolated from output 804 setting NMOS device 808 tologic LOW. When output WL 804 goes high, monitor node 810 is dischargedto ground, and device M0 812 is shut-off, thus isolating inputs 802, 803from output WL 804. The feedback loop precharges the rest of the nodesin the buffers via monitor node 810, and PMOSFET M13 815 is turned on,connecting the input x2_n 802 to node 810. Decoder 800 will not fireagain until x2_n 802 is reset to V_(dd), which usually happens when thesystem clock signal changes to logic LOW. Once x2_n 802 is logic HIGH,node 810 charges to V_(dd), with the assistance of PMOS device M14 818,and device M0 812 is turned on. This turns off PMOS device M13 815, thusisolating input x2_n 802 from the reset loop which employs node 810.Decoder 800 is now ready for the next input cycle.

[0116] Limited Swing Driver Circuit

[0117]FIG. 9 illustrates limited swing driver circuit 900 according toan aspect of the invention herein. In long word length memories, aconsiderable amount of power may be consumed in the data buses. Limitingthe voltage swing in such buses can decrease the overall powerdissipation of the system. This also can be true for a system where asignificant amount of power is dissipated in switching lines with highcapacitance. Limited-swing driver circuit 900 can reduce powerdissipation, for example, in high capacitance lines. When IN signal 902is logic HIGH, NMOS transistor MN1 904 conducts, and node 905 iseffectively pulled to ground. In addition, bitline 910 is dischargedthrough PMOSFET MP1 912. By appropriate device sizing, the voltage swingon bitline 910 can be limited to a desired value, when the inverter,formed by CMOSFETS MP2 914 and MN2 916, switches OFF PMOSFET MP1 912. Ingeneral, the size of circuit 900 is related to the capacitance(C_(bitline)) 918 being driven, and the sizes of MP2 914 and MN2 916. Inanother embodiment of this aspect of the present invention, limitedswing driver circuit includes a tri-state output enable, and aself-resetting feature. Tri-state functionality is desirable when datalines are multiplexed or shared. Although the voltage at memory cellnode 905 can swing to approximately zero volts, it is most desirablethat the bitline voltage swing only by about 200-300 mV.

[0118] Single-Ended Sense Amplifier with Sample-and-Hold Reference

[0119] In general, single-ended sense amplifiers are useful to savemetal space, however, existing designs tend not to be robust due totheir susceptibility to power supply and ground noise. In yet anotheraspect of the present invention, FIG. 10 illustrates a single-endedsense amplifier 1000, preferably with a sample-and-hold reference.Amplifier 1000 can be useful, for example, as a global sense amplifier,sensing input data. At the beginning of an operation, DataIn 1004 issampled, preferably just before the measurement begins. Therefore,supply, ground, or other noise will affect the reference voltage ofsense amplifier 1000 generally in the same way noise affects node to bemeasured, tending to increase the noise immunity of the sense amplifier1000. Both inputs 1010, 1011 of differential amplifier 1012 are at thevoltage level of DataIn 1004 when the activate signal (GWSELH) 1014 islogic LOW (i.e., at zero potential). At a preselected interval beforethe measurement begins, but before DataIn 1013 begins to change,activate signal (GWSELH) 1014 is asserted to logic HIGH, therebyisolating the input node 1002 of the transistor M162 1008. The DataInvoltage existing just before the measurement is taken is sampled andheld as a reference, thereby making the circuit substantiallyindependent of ground or supply voltage references. Transistors M1901025 and M187 1026 can add capacitance to the node 1021 where thereference voltage is stored. Transistor M190 1025 also can be used as apump capacitance to compensate for the voltage decrease at the referencenode 1021 when the activate signal becomes HIGH and pulls the source1002 of M162 1008 to a lower voltage. Feedback 1030 from output dataData_toLSA 1035, being transmitted to a local sense amplifier (notshown), is coupled with the source/drain of transistor M187 1026,actively adjusting the reference voltage at node 1021 by capacitivecoupling, thereby adjusting the amplifier gain adaptively.

[0120] Sense Amplifier with Offset Cancellation and Charge-share LimitedSwing Drivers

[0121] In yet another aspect of the present invention, a latch-typesense amplifier 1100 with dynamic offset cancellation is provided. Senseamplifier 1100 also may be useful as a global sense amplifier, and issuited for use in conjunction with hierarchical bitline sensing.Typically, the sensitivity of differential sense amplifiers can belimited by the offsets caused by inherent process variations for devices(“device matching”), and dynamic offsets that may develop on the inputlines during high-speed operation. Decreasing the amplifier offsetusually results in a corresponding decrease in the minimum bitline swingrequired for reliable operation. Smaller bitline swings can lead tofaster, lower power memory operation. With amplifier 1100, the offset onbitlines can be canceled by the triple PMOS precharge-and-balancetransistors M3 1101, M4 1102, M5 1103, which arrangement is known tothose skilled in the art. However, despite precharge-and-balancetransistors 1101-1103, an additional offset at the inputs of the latchmay exist. By employing balancing PMOS transistor (M14) 1110, any offsetthat may be present at the input of the latch-type differential senseamplifier can be substantially equalized. Sense amplifier 1100demonstrates a charge-sharing limited swing driver 1115. Global bitlines1150, 1151 are disconnected from sense amplifier 1100 when senseamplifier 1100 is not being used, i.e., in a tri-state condition. Senseamplifier 1100 can be in a precharged state if both input/output nodesare logic HIGH, i.e., if both of the PMOS drivers, M38 1130 and M29 1131are off (inputs at logic HIGH). A large capacitor, C₀ 1135, in senseamplifier 1100 can be kept substantially at zero volts by two seriesNMOS transistors, M37 1140 and M40 1141. The size of capacitor 1135 canbe determined by the amount of voltage swing typically needed on globalbitlines 1120, 1121.

[0122] When sense amplifier 1100 is activated, and bitlines 1150, 1151are logic HIGH, PMOS transistor M29 1131 is turned on and global bit_n1150 is discharged with a limited swing. When a bit to be read is logicLOW, PMOS transistor M38 1130 is turned on, and the global bit 1151 isdischarged with a limited swing. This charge-sharing scheme can resultin very little power consumption, because only the charge that causesthe limited voltage swing on the global bitlines 1150, 1151 isdischarged to ground. That is, there is substantially no “crowbar”current. Furthermore, this aspect of the present invention can be usefulin memories where the global bitlines are multiplexed for input andoutput.

[0123] Module-tier Memory Redundancy Implementation

[0124] In FIG. 12, memory structure 1200, composed of hierarchicalfunctional memory modules 1201 is preferred to have at least one or moreredundant memory rows 1202, 1204; one, or more redundant memory columns1206, 1208; or both, within each module 1201. It is preferred that theredundant memory rows 1202, 1204, and/or columns 1206, 1208 be paired,because it has been observed that bit cell failures tend to occur inpairs. Module-level redundancy, as shown in FIG. 12, where redundancy isimplemented using a preselected number of redundant memory rows 1202,1204, or redundant memory columns 1206, 1208, within memory module 1201,can be a very area-efficient approach provided the typical number of bitcell failures per module remains small. By implementing only a singlerow 1202 or a single column 1206 or both in memory module 1201, only oneadditional multiplexer is needed for the respective row or column.Although it may be simpler to provide redundant memory cell circuitsthat can be activated during product testing during the manufacturingstage, it may also be desirable to activate selected redundant memorycells when the memory product is in service, e.g., during maintenance oron-the-fly during product operation. Such activation can be effected bynumerous techniques and support circuitry which are well-known in theart.

[0125] Redundant Module Memory Redundancy Implementation

[0126] As shown in FIG. 13, memory redundancy also may be implemented byproviding redundant module 1301 to memory structure 1300, which iscomposed of primary modules 1304, 1305, 1306, 1307. Redundant module1301 can be a one-for-one replacement of a failed primary module, e.g,module 1304. In another aspect of the invention, redundant module 1301may be partitioned into smaller redundant memory segments 1310 a-d withrespective ones of segments 1310 a-d being available as redundant memorycells, for example, for respective portions of primary memory modules1304-1307 which have failed. The number of memory cells assigned to eachsegment 1310 a-d in redundant memory module 1301, may be a fixed number,or may be flexibly allocatable to accommodate different numbers offailed memory circuits in respective primary memory modules 1304-1307.

[0127] Memory Redundancy Device

[0128]FIG. 14 illustrates another aspect of the present invention whichprovides an implementation of row and column redundancy for a memorystructure such as memory structure 100 in FIG. 1, or memory structure300 in FIG. 3. This aspect of the present invention can be implementedby employing fuses that are programmable, for example, duringproduction. Examples of such uses include metal fuses that are blownelectrically, or by a focused laser; or a double-gated device, which canbe permanently programmed. Although the technique can be applied toprovide row redundancy, or column redundancy, or both, the presentdiscussion will describe column redundancy in which both inputs andoutputs may need the advantages of redundancy.

[0129]FIG. 14 shows an embodiment of this aspect of the invention hereinhaving four pairs of columns 1402 a-d with one redundant pair 1404. Itis desirable to implement this aspect of the present invention as pairsof lines because a significant number of RAM failures occur in pairs,whether column or row. Nevertheless, this aspect of the presentinvention also contemplates single line redundancy. In general, thenumber of fuses in fuse box 1403 used to provide redundancy can belogarithmically related to the number line pairs, e.g., column pairs:log₂ (number of column pairs), where the number of column pairs includesthe redundant pairs as well. Because fuses tend to be large, theirnumber should be minimized, thus the logarithmic relation isadvantageous. Fuse outputs 1405 are fed into decoder circuits 1406 a-d,e.g., one fuse output per column pair. A fuse output creates what isreferred to herein as a “shift pointer”. The shift pointer indicates theshift signal in the column pair to be made redundant, and subsequentcolumn pairs can then be inactivated. It is desirable that the signals1405 from fuse box 1410 are decoded to generate shift signal 1412 a-d ateach column pair. When shift signal 1412 a-d for a particular columnpair 1402 a-d location is selected, as decoded from fuse signals 1405,shift pointer 1412 a-d is said to be pointing at this location. Theshift signals for this column, and all subsequent columns to the rightof the column of pair shift pointer also become inactive.

[0130] This aspect of the present invention can be illustratedadditionally in FIG. 15A and FIG. 15B, by way of the aforementionedconcept of “shift pointers.” In FIG. 15A, three column pairs 1501, 1502,1503, and one redundant column pair 1504 are shown. The shift procedureis conceptually indicated by way of “line diagrams”. The top lines1505-1508 of the line diagrams are representative of columns 1501-1504within the memory core while bottom line pairs 1509-1511 are the datainput/output pairs from the input/output buffers. When a shift signal,such as a signal 1405 in FIG. 14, for a particular column pair 1501-1503is logical LOW, it is preferred that the data in 1509-1511 be connectedto respective column 1501-1503 directly above it by multiplexers. FIG.15B is illustrative of having a failed column state. When shift signalis logical HIGH, such as a signal 1405 in FIG. 14, a failed column isindicated, such as column 1552. Active columns 1550, 1551 remainunfaulted, and continue to receive their data via I/O lines 1554, 1555.However, because column 1552 has failed, data from I/O buffer 1556 canbe multiplexed to the redundant column pair 1553. Diagrammatically, itappears that data in are shifted left while data out from the memorycore columns are shifted right. By adjusting the location of the shiftpointer, which generally is determined by the state of the fuses, theunused redundant column pair can be shifted to coincide with anonfunctional column, e.g., column 1552, thereby repairing the columnfault and boosting the fully functional memory yield.

[0131] Selector for Redundant Memory Circuits

[0132]FIG. 16 illustrates yet another aspect of the present invention,in which selector 1600 is adapted to provide a form of redundancy.Selector 1600 can include a primary decoder circuit 1605, which may be aglobal word line decoder, which is coupled with a multiplexer 1610. MUX1610 can be activated by a redundancy circuit 1620, which may be a fusesystem, programable memory, or other circuit capable of providing anactivation signal 1630 to selector 1600 via MUX 1610. Selector 1600 issuitable for implementing module-level redundancy, such as thatdescribed relative to module 1200 in FIG. 12, which may be rowredundancy or column redundancy for a given implementation. In theordinary course of operation, input word line signal 1650 is decoded indecoder circuit 1605 and, in the absence of a fault on local word line1670, the word line signal is passed to first local line 1680. In theevent a fault is detected, MUX 1610, selects second local line 1660,which is preferred to be a redundant word line.

[0133] Fast Decoder with Row Redundancy

[0134]FIG. 17 illustrates a preferred embodiment of selector 1600 inFIG. 16, in the form of decoder 1700 with row redundancy as realized ina hierarchical memory environment. Decoder 1700 may be particularlysuitable for implementing module-level redundancy, such as thatdescribed relative to module 1200 in FIG. 12. Global decoder 1700, canoperate similarly to the manner of asynchronously-resettable decoder 800of FIG. 8. In general, decoder 1700 can be coupled with a first,designated memory row, and a second, alternative memory row. Althoughthe second row may be a physical row adjacent the first memory row, andanother of the originally designated rows of the memory module, thesecond row also may be a redundant row which is implemented in themodule. Although row decoder 1700 decodes the first memory row undernormal operations, it also is disposed to select and decode the secondmemory row in responsive to an alternative-row-select signal. Where thesecond row is a redundant row, it may be more suitable to deem theselection signal to be a “redundant-row-select” signal. Theaforementioned row select signals are illustrated as inputs 1701 and1702.

[0135] Thus, when input 1701 or 1702 is activated, decoder 1700transfers the local word line signal, usually output on WL 1706, to beoutput on xL_Next 1705, which is coupled with an adjacent word line. Ingeneral, when a word line decoder, positioned at a particular locationin a memory module, receives a shift signal, the remaining decoderssubsequent to that decoder also shift, so that the last decoder in thesequence shifts its respective WL data to a redundant word line. Using atwo-dimensional conceptual model where a redundant row is at the bottomof a model, this process may be described as having a fault at aparticular position effect a downward shift of all local word lines atand below the position of the fault. Those local word lines above theposition of the fault can remain unchanged.

[0136] Hybrid Single Port and Dual Port (R/W) Functionality

[0137] Hierarchical memory module implementations realize significanttime savings due in part to localized functionality. Signal propagationtimes at the local module tier tend to be substantially less than thetypical access time of a larger memory structure, even those employingexisting paged, banked, and segmented memory array, and register fileschemes. Indeed, both read and write operations performed at thefundamental module tier can occur within a fraction of the overallmemory structure access time. Furthermore, because bitline sensing, inaccordance with the present invention, is power-conservative, and doesnot result in a substantial decay of precharge voltages, the bitlinevoltage levels after an operation tend to be marginally reduced. As aresult, in certain preferred embodiments of the present invention, it ispossible to perform two operations back-to-back without an interveningpre-charge cycle, and to do so within a single access cycle of theoverall memory structure.

[0138] Therefore, although a memory device may be designed as to besingle-port device, a preferred memory module embodiment functionssimilarly to a two-port memory device, which can afford such anembodiment a considerable advantage over prior art memory structures ofcomparable overall memory size.

[0139]FIG. 18 illustrates one particular embodiment of this aspect ofthe present invention, in memory structure 1800, where both localbitline sensing and local word line decoding are used, as describedabove. Memory structure 1800 includes memory module 1805 which iscoupled with local word line decoder 1815 and local bit sense amplifier1820. Within memory module 1805 are a predefined number of memory cells,for example, memory cell 1825, which is coupled with local word linedecoder 1815 via local word line 1810, and local bit sense amplifier1820 via local bitlines 1830. With typical single-port functionality,local bitlines 1830 are precharged prior to both READ and WRITEoperations. During a typical READ operation, predecoder 1835 activatesthe appropriate global word line decoder 1840, which, in turn, activateslocal word line decoder 1815. Once local word line decoder 1815determines that associated memory cell 1825 is to be evaluated, it opensmemory cell 1825 for evaluation, and activates local bit sense amplifier1820. At the end of the local sensing period, local bit sense amplifier1820 outputs the sensed data value onto global bitlines 1845. Afterglobal sense amplifier 1850 senses the data value, the data is output tothe I/O buffer 1855. If a WRITE operation is to follow the READoperation, a typical single-port device would perform another prechargeoperation before the WRITE operation can commence.

[0140] In this particular embodiment of dual-port functionality, thepredecoding step of a subsequent WRITE operation can commenceessentially immediately after local bitline sense amplifier 1820completes the evaluation of memory cell 1825, that is, at the inceptionof sensing cycle for global sense amplifier 1850, and prior to the databeing available to I/O buffer 1855. Thus, during the period encompassingthe operation of global sense amplifier 1850 and I/O buffer 1855, andwhile the READ operation is still in progress, predecoder 1835 canreceive and decode the address signals for a subsequent WRITE operation,and activate global word line decoder 1840 accordingly. In turn, globalword line decoder 1840 activates local word line 1815 in anticipation ofthe impending WRITE operation. As soon as the datum is read out of I/Obuffer 1855, the new datum associated with the WRITE cycle can beadmitted to I/O buffer 1855 and immediately written to, for example,memory cell 1825, without a prior precharge cycle. In order to providethe memory addresses for these READ and WRITE operations in a mannerconsistent with this embodiment of the invention, it is preferred thatthe clocking cycle of predecoder 1810 be faster than the access cycle ofthe overall memory structure 1800. For example, it may be desirable toadapt the predecoding clock cycle to be about twice, or perhaps greaterthan twice, the nominal access cycle for structure 1800. In this manner,a PRECHARGE-READ-WRITE operation can be performed upon the same memorycell within the same memory module in less than one access cycle,thereby obtaining dual-port functionality from a single port device. Italso is contemplated that the aforementioned embodiment can be adaptedto realize three or more operations within a single access cycle, aspermitted by the unused time during an access cycle.

[0141] Fortuitously, the enhanced functionality described above isparticularly suited to large memory structures with comparatively smallconstituent modules, where the disparity between global and local accesstimes is more pronounced. Moreover, in environments where delays due tosignal propagation across interconnections, and to signal propagationdelays through co-embedded logic components may result in sufficientidle time for a memory structure, this enhanced functionality mayadvantageously make use of otherwise “wasted” time.

[0142]FIG. 19 illustrates high precision delay measurement (HPDM)circuit 1900, according to one aspect of the present invention, whichcan provide timing measurements of less than that of a single gatedelay, relative to the underlying technology. These measurements can be,for example, of signal delays and periods, pulse widths, clock skews,etc. HPDM circuit 1900 also can provide pulse, trigger, and timingsignals to other circuits, including sense amplifiers, word linedecoders, clock devices, synchronizers, state machines, and the like.Indeed, HPDM circuit 1900 is a measurement circuit of widespreadapplicability. For example, HPDM circuit 1900 can be implemented withina high-performance microprocessor, where accurate measurement ofinternal time intervals, perhaps on the order of a few picoseconds, canbe very difficult using devices external to the microprocessor. HPDMcircuit 1900 can be used to precisely measure skew between and amongsignals, and thus also can be used to introduce or eliminate measuredskew intervals. HDPM circuit 1900 also can be employed to characterizethe signals of individual components, which may be unmatched, orpoorly-matched components, as well as to bring such components intosubstantial synchrony. Furthermore, HPDM circuit 1900 can advantageouslybe used in register files, transceivers, adaptive circuits, and a myriadof other applications in which precise interval measurement is desirablein itself, and in the context of adapting the behavior of components,circuits, and systems, responsive to those measured intervals.

[0143] Advantageously, HPDM circuit 1900 can be devised to be responsiveto operating voltage, design and process variations, design rulescaling, etc., relative to the underlying technology, including, withoutlimitation, bipolar, nMOS, CMOS, BiCMOS, and GaAs technologies. Thus, anHPDM circuit 1900 designed to accurately measure intervals relevant to1.8 micron technology will scales in operation to accurately measureintervals relevant to 0.18 micron technology. Although HPDM circuit 1900can be adapted to measure fixed time intervals, and thus remainindependent of process variations, design rule scaling, etc., it ispreferred that HPDM circuit 1900 be allowed to respond to the technologyand design rules at hand. In general, the core of an effective HPDMcircuit capable of measuring intervals on the order of picoseconds, canrequire only a few scores of transistors which occupy a minimalfootprint. This is in stark contrast to its counterpart in thehuman-scale domain, i.e., a an expensive, high-precision handheld, orbench side, electronic test device.

[0144] One feature of HPDM circuit 1900 is modified ring oscillator1905. As is well-known in the art of ring oscillators, the oscillationperiod, T₀, of a ring oscillator having N stages is approximately equalto 2NT_(D), where T_(D) is the large-signal delay of the gate/inverterof each stage. The predetermined oscillation period, T₀, can be chosenby selecting the number of gates to be employed in the ring oscillator.In general, T_(D) is a function of the rise and fall times associatedwith a gate which, in turn, are related to the underlying parametersincluding, for example, gate transistor geometries and fabricationprocess. These parameters are manipulable such that T_(D) can be tunedto deliver a predetermined gate delay time. In a preferred embodiment ofthe present invention in the context of a specific embodiment of ahierarchical memory structure, it is desirable that the parameters berelated to a CMOS device implementation using 0.18 micron (μm) designrules. However, a skilled artisan would realize that HPDM circuit 1900is not limited thereto, and can be employed in other technologies,including, without limitation, bipolar, nMOS, CMOS, BiCMOS, GaAs, andSiGe technologies, regardless of design rule, and irrespective ofwhether implemented on Si substrate, SOI and its variants, etc.

[0145] Although exemplary HPDM circuit 1900 employs seven (7) stage ringoscillator 1905, a greater or lesser number of stages may be used,depending upon the desired oscillation frequency. In this example, ringoscillator 1905 includes NAND gate 1910, the output of which beingdesignated as the first stage output 1920; and six inverter gates,1911-1916, whose outputs 1921-1926 are respectively designated as thesecond through seventh stage outputs.

[0146] In addition to ring oscillator 1905, HPDM circuit 1900 caninclude memory elements 1930-1937, each of which being coupled with apreselected oscillator stage. The selection and arrangement of memoryelements 1930-1937, make it possible to measure a minimum time quantum,T_(L), which is accurate to about one-half of a gate delay, that is,T_(L)≈T_(D)/2. The maximum length of time, T_(M), that can usefully bemeasured by HPDM circuit 1900 is determinable by selecting one or morememory devices, or counters, to keep track of the number of oscillationcycles completed since the activation of oscillator 1905, for example,by ENABLE signal 1940. Where the selected counter is a single 3-bitdevice, for example, up to eight (8) complete cycles through oscillator1905 can be detected, with each cycle being completed in T₀ time.Therefore, using the single three-bit counter as an example, T_(M)≈8T₀.The remaining memory elements 1932-1937 can be used to indicate thepoint during a particular oscillator cycle at which ENABLE signal 1940was deactivated, as determined by examining the respective states ofgiven memory elements 1932-1937 after deactivation of oscillator 1905.

[0147] In HPDM circuit 1900, it is preferred that a k-bit positiveedge-triggered counter (PET) 1930, and a k-bit negative edge-triggeredcounter (NET) 1931, be coupled with first stage output 1920. Further, itis preferred that a dual edge-triggered counter (DET) 1932-1937 becoupled with respective outputs 1921-1925 of Oscillator 1905. In aparticular embodiment of the invention, PET 1930 and NET 1931 are eachselected to be three-bit counters (i.e., k=3), and each of DET 1932-1937are selected to be one-bit counters (latches). An advantage of usingdual edge detection in counters 1932-1937 is that the edge of aparticular oscillation signal propagating through ring oscillator 1905can be registered at all stages, and the location of the oscillationsignal at a specific time can be determined therefrom. Because apropagating oscillation signal alternates polarity during sequentiallysubsequent passages through ring oscillator 1905, it is preferred toemploy both NET circuit 1930 and PET 1931, and that the negative edge ofa particular oscillation signal be sensed as the completion of the firstlooping event, or cycle, through ring oscillator 1905.

[0148] The operation of HPDM circuit 1900 can be summarized as follows:with EnableL signal 1904 asserted HIGH, ring oscillator 1905 is in theSTATIC mode, so that setting ResetL signal 1906 to LOW resets counters1930-1937. By setting StartH signal 1907 to HIGH, sets RS flip-flop 1908which, in turn, sets ring oscillator 1905 to the ACTIVE mode bypropagating an oscillation signal. Each edge of the oscillation signalcan be traced by identifying the switching activity at each stage output1920-1926. PET 1930 and NET 1931, which sense first stage output 1920identify and count looping events. It is preferred that the maximumdelay to be measured can be represented by the maximum count of PET 1930and NET 1931, so that the counters do not overflow. To stop thepropagation of the oscillation signal through ring oscillator 1905,StopL signal 1909 is set LOW, RS flip-flop 1908 is reset, and ringoscillator 1905 is returned to the STATIC mode of operation. Also, thedata in counters 1930-1937 are isolated from output stages 1920-1926 bysetting enL signal 1950 to LOW and enH signal 1951 to HIGH. The digitaldata is then read out through ports lpos 1955, lneg 1956, and del 1957.With knowledge of the average stage delay, the digital data then can beinterpreted to provide an accurate measurement, in real time units, ofthe interval during which ring oscillator 1905 was in the ACTIVE mode ofoperation. HPDM circuit 1900 can be configured to provide, for example,a precise clock or triggering signal, such as TRIG signal 1945, afterthe passage of a predetermined quantum of time. Within the context of amemory system, such quantum of time can be, for example, the timenecessary to sense the state of a memory cell, to keep active awordline, etc.

[0149] The average stage delay through stages 1910-1916 can bedetermined by operating ring oscillator 1905 for a predeterminedaveraging time by asserting StartH 1907 and StopL 1909 to HIGH, therebyincrementing counters 1930-1937. In a preferred embodiment of thepresent invention, the overflow of NET 1931 is tracked, with eachoverflow event being indicative of 2^(k) looping events through ringoscillator 1905. It is preferred that this tracking be effected by adivider circuit, for example, DIVIDE-BY-64 circuit 1953. At the end ofthe predetermined averaging time, data from divider 1953 may be read outthrough port RO_div64 1954 as a waveform, and then analyzed to determinethe average oscillator stage delay. However, a skilled artisan wouldrealize that the central functionality of HPDM circuit 1900, i.e., toprovide precise measurement of a predetermined time quantum, wouldremain unaltered if DIVIDE-BY-64 circuit 1953, or similar dividercircuit, were not included therein.

[0150] HPDM circuit 1900 can be used for many timing applicationswhether or not in the context of a memory structure, for example, toprecisely shape pulsed waveforms and duty cycles; to skew, deskew acrossone or more clocked circuits, or to measure the skew of such circuits;to provide high-precision test data; to indicate the beginning, end, orduration of a signal or event; and so forth. Furthermore, HPDM circuit1900 can be applied to innumerable electronic devices other than memorystructures, where precise timing measurement is desired.

[0151] Accurate self-timed circuits are important features of robust,low-power memories. Replica bitline techniques have been described inthe prior art to match the timing of control circuits and senseamplifiers to the memory cell characteristics, over wide variations inprocess, temperature, and operation voltage. One of the problems withsome prior art schemes is that split dummy bitlines cluster word-linestogether into groups, and thus only one word-line can be activatedduring a memory cycle. Before a subsequent activation of a word-linewithin the same group, the dummy bitlines must be precharged, creatingan undesirable delay. The diffusion replica delay technique of thepresent invention substantially matches the capacitance of a dummybitline by using a diffusion capacitor, preferably for each row. Someprior art techniques employed replica bit-columns which can add toundesirable operational delays. FIG. 20 illustrates the diffusionreplica timing circuit 2000 which includes transistor 2005 and diffusioncapacitance 2010. It is desirable that transistor 2005 be an NMOSFETtransistor which, preferably, is substantially identical to an accesstransistor chain, if such is used in the memory cells of the memorystructure (not shown). It also is desirable that the capacitance ofdiffusion capacitor 2010 is substantially matched to the capacitance ofthe associated bitline (not shown). This capacitance can be apredetermined ratio of the total bitline capacitance, with the ratio ofthe diffusion capacitance to total bitline capacitance remainingsubstantially constant over process, temperature and voltage variations.The total bitline capacitance can include both the bitline metal anddiffusion capacitances. In this fashion, all rows in a memory devicewhich use timing circuit 2000 can be independently accessible withsubstantially fully-operation self-timing, even when another row in thesame memory module has been activated, and is not yet precharged. Thus,write-after-read operations may be multiplexed into a memory modulewithout substantial access time or area penalties. Thus, it is desirableto employ diffusion replica delay circuit 2000 in a memory structuresuch as memory structure 1800, described in FIG. 18. Diffusion replicadelay circuit 2000 can be used to determine the decay time of a bitlinebefore a sense amplifier is activated, halting the decay on the bitline.In this manner, bitline decay voltage can be limited to a relativelysmall magnitude, thus saving power and decreasing memory access time.Furthermore, timing circuit 2000 can be used to accurately generate manytiming signals in a memory structure such as structure 1800 in FIG. 18,including, without limitation, precharge, write, and shut-off timingsignals.

[0152]FIG. 21 illustrates an embodiment of the diffusion replica delaycircuit 2000 in FIG. 20. Word-line activation of a memory cell frequencyis pulsed to limit the voltage swing on the high capacitance bitlines,in order to minimize power consumption, particularly in wide word lengthmemory structures. In order to accurately control the magnitude of abitline voltage swing, dummy bitlines can be used. It is desirable thatthese dummy bitlines have a capacitance which is a predefined fractionof the actual bitline capacitance. In such a device, the capacitanceratio between dummy bitlines and real bitlines can affect the voltageswing on the real bitlines. In prior art devices using dummy bitlines, aglobal dummy bitline for a memory block having a global reset loop hasbeen utilized. Such prior art schemes using global resetting tends todeliver pulse widths of a duration substantially equivalent to the delayof global word-line drivers. Such an extend pulse width allows for abitline voltage swing which can be in excess of what actually isrequired to activate a sense amplifier. This is undesirable in fastmemory structures, because the additional, and unnecessary, voltageswing translates into a slower structure with greater powerrequirements. In one aspect of the present invention, dummy bitlines arepreferably partitioned such that the local bitlines generally exhibit asmall capacitance and a short discharge time. Word-line pulse signals ofvery short duration (e.g., 500 ps or less) are desirable in order tolimit the bitline voltage swing. It also may be desirable to providelocal reset of split dummy bitlines to provide very short word-linepulses. Replica word-line 2110 can be used to minimize the delay betweenactivation of memory cell 2120 and related sense amplifier 2130. Suchlocal signaling is preferred over global signal distribution onrelatively long, highly capacitive word-lines. Word-line 2140 activatesdummy cell 2150 along with associated memory cell 2120, which is to beaccessed. Dummy cell 2150 can be part of dummy column 2160 which may besplit into small groups (for example, eight or sixteen groups). The sizeof each split dummy group can be changed to adjust the voltage swing onthe bitline. When a dummy bitline is completely discharged, reset signal2170 can be locally generated which pulls word-line 2140 substantiallyto ground.

[0153]FIG. 22A illustrates controlled voltage swing data bus circuit(CVS) 2200 which can be useful in realizing lower power, high speed, anddense interconnection buses. CVS 2200 can reduce bus power consumptionby imposing a limited, controlled voltage swing on bus 2215. In anessential configuration, CVS 2000 can include inverter 2205, PMOS passtransistor T2 2210, and one nMOS discharge transistor, such astransistor T1 a 2205 a. Both transistors T1 a 2205 a, and T2 2210 can beprogrammed to control the rate and extent of voltage swings on bus 2215such that a first preselected bus operational characteristic is providedin response to input signal 2220 a. Additional discharge transistors T1b 2205 b and T1 c 2205 c can be coupled with pass transistor T2 2210,and individually programmed to respectively provide a second preselectedbus operational characteristic, as well as a third preselected busoperational characteristic, responsive to respective input signals 2220b, 2220 c. The preselected bus operational characteristic can be forexample, the rate of discharge of the bus voltage through the respectivedischarge transistor T1 a 2205 a, T1 b 2205 b, and T1 c 2205 c, suchthat bus 2215 is disposed to provide encoded signals, or multilevellogic, thereon. For example, as depicted in FIG. 22A, CVS 2200 canprovide three distinct logic levels. Additional discharge transistors,programmed to provide yet additional logic levels also may be used.Thus, it is possible for bus 2215 to replace two or more lines.Concurrently with effecting a reduction in power consumption, thelimited bus voltage swing advantageously tends to increase the speed ofthe bus.

[0154]FIG. 22B illustrates a bidirectional data bus transfer circuit(DBDT) 2250 which employs cross-linked inverters I1 2260 and I2 2270 tocouple BUS 1 2252 with BUS 2 2254. It is desirable to incorporate aclocked charge/discharge circuit with DBDT 2250. Coupled with inverterI1 2260 is clocked charge transistor MPC1 2266 and clocked dischargetransistor MNC1 2268.

[0155] Similarly, inverter I2 2270 is coupled with clocked chargetransistor MPC2 2276 and clocked discharge transistor MNC2 2278.Transistors MPC1 2266, MNC1 2268, MPC2 2276, and MNC2 2278 are preferredto be driven by clock signal 2280.

[0156] Beginning with clock signal 2280 going LOW, charge transistorsMPC1 2266 and MPC2 2276 turn ON, allowing BUS 1 input node 2256 and BUS2 input node 2258 to be precharged to HIGH. Additionally, dischargetransistors MNC1 2268 and MNC2 2278 are turned OFF, so that nosubstantial discharge occurs. By taking input nodes 2256, 2258 to HIGH,respective signals propagate through, and are inverted by inverters I12260 and I2 2270 providing a LOW signal to BUS 1 pass transistor MP122262 and BUS 2 pass MP22 2272, respectively, allowing the signal on BUS1 2252 to be admitted to input node 2256, and then to pass through toBUS2 input node 2258 to BUS 2 2254, and vice versa. When clock signal2280 rises to HIGH, both charge transistors MPC1 2266 and MPC2 2276 turnOFF, and discharge transistors MNC1 2268 and MNC2 2278 turn ON, latchingthe data onto BUS 1 2252 and BUS 2 2254. Upon the next LOW phase ofclock signal 2280, a changed signal value on either BUS 1 2252 or BUS 22254 will propagate between the buses.

[0157] Many alterations and modifications may be made by those havingordinary skill in the art without departing from the spirit and scope ofthe invention. Therefore, it must be understood that the illustratedembodiments have been set forth only for the purposes of example, andthat it should not be taken as limiting the invention as defined by thefollowing claims. The following claims are, therefore, to be read toinclude not only the combination of elements which are literally setforth but all equivalent elements for performing substantially the samefunction in substantially the same way to obtain substantially the sameresult. The claims are thus to be understood to include what isspecifically illustrated and described above, what is conceptuallyequivalent, and also what incorporates the essential idea of theinvention.

1. A limited swing driver, comprising: a. a pass transistor coupledbetween a memory cell and an associated bitline; b. an inverter havingan input and an output, the output coupled to the gate of the passtransistor, selectively driving the pass transistor thereby, theinverter input being coupled with the memory cell, the coupling ofinverter input and the memory cell forming a memory node; and c. adischarge transistor coupled between the memory node and ground, thedischarge transistor being driven by an input on the dischargetransistor gate, the discharge transistor being programmed to produce alimited swing voltage at the memory node.
 2. The limited swing driver ofclaim 1, wherein the limited swing voltage is less than about 350 mV. 3.The limited swing driver of claim 2, wherein the limited swing voltageis between about 300 mV and about 200 mV.
 4. The limited swing voltagedriver of claim 1, further comprising at least one of: a. a tri-stateoutput enable isolating the memory node from the bitline, the bitlinebeing one of a shared bitline and a multiplexed bitline; and b. aself-reset circuit resetting the driver to a predetermined signal state.